Photolithographically-patterned variable capacitor structures and method of making

ABSTRACT

A new type of high-Q variable capacitor includes a substrate, a first electrically conductive layer fixed to the substrate, a dielectric layer fixed to a portion of the electrically conductive layer, and a second electrically conductive layer having an anchor portion and a free portion. The anchor portion is fixed to the dielectric layer and the free portion is initially fixed to the dielectric layer, but is released from the dielectric layer to become separated from the dielectric layer, and wherein an inherent stress profile in the second electrically conductive layer biases the free portion away from the a dielectric layer. When a bias voltage is applied between the first electrically conductive layer and the second electrically conductive layer, electrostatic forces in the free portion bend the free portion towards the first electrically conductive layer, thereby increasing the capacitance of the capacitor.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation of U.S. application Ser. No. 09/573,363 filed May17, 2000 and now U.S. Pat. No. 6,396,677, the contents of which areincorporated herein by reference.

This application is related to U.S. application Ser. No. 09/573,815,filed May 17, 2000, U.S. application Ser. No. 09/975,358 filed Oct. 11,2001, U.S. application Ser. No. 09/591,262 filed Jun. 9, 2000, and U.S.application Ser. No. 10/004,819 filed Dec. 7, 2001.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention generally relates to photolithographically-patterned,out-of-plane coil structures for use in integrated circuits, circuitboards and other devices.

2. Description of Related Art

Standard bonding techniques for electrically connecting integratedcircuits, or chips, to a circuit board or other device include wirebonding, tab bonding, and solder-bump flip-chip bonding. FIG. 1 shows acontact pad 3 formed on a chip 2 wire bonded to a corresponding contactpad 3 formed on a substrate 1. The contact pads 3 are electricallyconnected, or bonded, by a wire 4. Since the chip 2 typically has tensor even hundreds of the contact pads 3, wire bonding each contact pad 3on the chip 2 to the corresponding contact pad 3 on the substrate 1 islabor intensive, expensive and slow. Further, the contact pads 3 must belarge enough to accommodate both the wire 4 and the accuracy of the wirebonding device used to create the wire bond. Therefore, the contact pads3 are made larger than otherwise necessary to compensate for the sizelimitations of wire 4 and the wire bonding device.

FIG. 2 shows the contact pad 3 formed on the chip 2 tab bonded to thecorresponding contact pad 3 on the substrate 1. A flexible substrate 5having conductive lines formed on its lower surface is forced againstthe contact pads 3. A layer of anisotropic adhesive (not shown) isplaced between the contact pads 3 and the flexible substrate 5. When theflexible substrate 5 is pressed against the contact pads 3, theanisotropic adhesive and the conductive lines formed on the flexiblesubstrate 5 cooperate to complete the electrical connection between thecontact pads 3. Like wire bonding, tab bonding suffers from yield lossand high cost. Irregularities in the heights of the contact pad 3 resultin non-uniform contacting force pressing the flexible substrate 5against the contact pads 3. The non-uniform contacting force means thatsome contact pads 3 will not be properly bonded to the flexiblesubstrate 5.

Another conventional method for bonding the contact pads 3 formed on thechip 2 to the contact pads 3 formed on the substrate 1 or to some otherdevice is solder-bump flip-chip bonding. FIG. 3 shows the chip 2inverted with the contact pads 3 facing toward the substrate 1. The name“flip-chip” derives from the inversion of the chip 2, since the chip 2is “flipped over” with the contacts pads 3 facing the substrate 1, incontrast to both tab bonding and wire bonding where the contact pads 3on the chip 2 face away from the substrate 1. In standard flip-chipbonding, solder bumps 6 are formed on the contact pads 3 on thesubstrate 1. The electrical connection between the corresponding contactpads 3 is completed by pressing the contact pads 3 on the chip 2 againstthe solder bumps 6.

Flip-chip bonding is an improvement over both wire bonding and tabbonding. The relatively soft solder bumps 6 tend to permanently deformwhen the chip 2 is pressed down against the solder bumps 6. Thisdeformation of the solder bumps 6 compensates for some irregularity inthe heights of the contact pads 3 and any uneven contacting pressureforcing the chip 2 against the solder bumps 6.

However, flip-chip bonding does suffer from both mechanical and thermalvariations in the solder bumps 6. If the solder bumps 6 are not uniformin height or if the substrate 1 is warped, contact between the contactpads 3 and the solder bumps 6 can be broken. Also, if the contactingpressure forcing the chip 2 down on the solder bumps 6 is uneven,contact between some contact pads 3 and corresponding solder bumps 6 canfail.

FIG. 4 shows a standard technique for establishing a temporaryelectrical contact between two devices. A probe card 7 having aplurality of probe needles 8 contacts the contact pads 3 by physicallypressing the probe needles 8 against the contact pads 3. The physicalcontact between the probe needles 8 and the contact pads 3 creates anelectrical connection between the probe needles 8 and the lines 9 formedon the substrate 1.

The probe cards 7 are generally used to create only temporary contactsbetween the probe needles 8 and the contact pads 3, so that the device10 can be tested, interrogated or otherwise communicated with. Thedevice 10 can be a matrix of display electrodes which are part of anactive-matrix liquid crystal display. Testing of the devices 10, such asliquid crystal display electrode matrices, is more thoroughly describedin an application JAO 34053 to the same inventor, co-filed andco-pending herewith and herein incorporated by reference.

The probe cards 7 have many more applications than only for testingliquid crystal displays. Any device 10 having numerous and relativelysmall contact pads 3, similar to those found on the chip 2, can betested using the probe card 7. However, standard techniques forproducing the probe card 7 are time consuming and labor-intensive. Eachprobe card 7 must be custom-made for the particular device 10 to betested. Typically, the probe needles 8 are manually formed on the probecard 7. Because the probe cards 7 are custom-made and relativelyexpensive, the probe cards 7 are not typically made to contact all ofthe contact pads 3 on the device 10 at one time. Therefore, onlyportions of the device 10 can be communicated with, tested orinterrogated at any one time, requiring the probe card 7 be moved toallow communication, testing or interrogation of the entire device 10.

The probe cards 7 are also used to test the chips 2 while the chips 2are still part of a single-crystal silicon wafer. One such probe card 7is formed by photolithographic pattern plated processing, as disclosedin Probing at Die Level, Corwith, Advanced Packaging, February, 1995,pp. 26-28. Photolithographic pattern plated processing produces probecards 7 which have essentially the same design as the standard probecard 7. However, this new type of processing appears to automate themethod for producing probe needles 8, thus avoiding manually forming theprobe needles 8. Also, this article discloses a probe card 7 which isbent at the end nearest the probe needles 8, as shown in FIG. 5. Thebend in the probe card 7 allows the probe needles 8 to contact thecontact pad 3 at an angle. As the probe card 7 pushes the probe needles8 into the contact pads 3, a mechanical scrubbing action occurs whichallows the probe needles 8 to break through the oxide formed on the topsurface of the contact pad 3.

All of the standard probe cards 7, however, are limited to testingcontact pads 3 which are arranged in a linear array. Also, the standardprobe cards 7 are sensitive to variations in the height of the contactpads 3 on the substrate 1, irregularities or warping of the substrate 1,and temperature variations.

The integration of small inductors on silicon substrates has been thesubject of intense worldwide research for more than 15 years. Thiseffort is driven by the desire to integrate coils on silicon and galliumarsenide integrated circuits (ICs). The structures proposed so far,however, have been variations of devices in which, due to technologicalconstraints, the coil windings have almost always been implemented asspirals parallel to the underlying substrate.

These in-plane architectures have two major drawbacks. When made on asubstrate that is slightly conducting such as silicon, the coil magneticfields induce eddy currents in the underlying substrate. These currentscause resistive dissipation that contributes to the coil losses. Thesecond problem arises when the coil is operated at high frequencies,where skin and proximity effects force the coil current to flow alongthe outer surfaces of the conductor. The “skin depth” is about 2 to 3 μmfor typical conductors at frequencies of interest for wirelesscommunication, for example, 900 MHz, 1.9 GHz and 2.4 GHz. The ACresistance of the coil conductor becomes appreciably higher than its DCresistance because the cross section of the conductor is not fully used.

FIG. 31 shows the current distribution in in-plane coils operated athigh frequencies. Darker shading in the coil indicates a higher currentdensity. The disk-shaped coil shown in FIG. 31a has a currentdistribution that is concentrated at the outer edges of the windingwire. Therefore, widening the conductor simply increases the unusedportion of the conductor and does not reduce the AC resistance. Thissituation may be compared to the typical discrete component,out-of-plane coil of FIG. 31b, where the AC resistance can be reduced bysimply making the conductors wider.

Solutions have been proposed and tried in the past to address thedrawbacks associated with in-plane inductor coils. Eddy currents can bereduced, for example, by etching away the substrate underneath the coil.However, this approach is not practical as it sacrifices structuralintegrity and destroys existing electronic circuitry on the siliconsubstrate. To reduce the AC resistance of the device in FIG. 31a, theconductor can be made very thick using micromachining techniques such asLIGA (see A. Rogner et al., “The LIGA technique—what are the newopportunities,” J. Micromech. Microeng., vol.2, pp. 133-140, 1992.).However, processing high aspect ratio structures is difficult andexpensive.

Various out-of-plane techniques have been suggested. For example,Chukwunenye Stanley Nnebe, in “A Mechanically-raised MicromachinedVariable Inductor Coil” (www.ee.cornell.edu/MENG/Abstracts/tien.htm)describes an out-of plane variable inductor structure. The structure isinitially gold-metallized strips of polysilicon on the surface plane ofthe substrate, which are then raised and fastened via a hinging systemto form a triangular geometry upon contact. After the setup of the coilis completed, the slider representing the magnetic core can then beactivated through an impact system that is controlled by four comb-driveresonators (two comb-drive resonators for each direction of motion). Theinsertion of the magnetic core through the coils would influence themagnetic flux developed around the coils and, thus, would vary theinductance accordingly. The tuning range of the inductor is set by thiseffect, and reliable data may be obtained by carefully controlling thefour resonators that actuate the slider causing it to move a finitedistance through the coils. Such a technique is fairly complicated tomicromachine and requires additional components on valuable chip realestate.

Robert Marcus et al. in International PCT Application number WO 99/18445filed Oct. 2, 1998, titled Michromachined Element and Method ofFabrication Thereof, discloses a coiled structure that is formed bydepositing two layers of material having different coefficients ofthermal expansion on a sacrificial layer, removing the sacrificiallayer, then heating the cantilevered structure until it curls partiallyupon itself. Coil closure is achieved by patterning a tethered end tothe tip of the cantilevered structure. When the sacrificial layer isremoved and the cantilever heated, the cantilever curls on itself,causing the tethered end to twist. Such a method and structure, however,is impractical for creating arrays of densely packed, integratedmicro-inductors and other structures on silicon substrates.

Low-loss inductors that can be integrated on chip are most desirable inwireless communication devices such as cellular phones, pagers, GPSreceivers, warehouse management RF identification tags, wirelesscomputer LANs, personal digital assistants, and satellitetelecommunication. Small portable devices, in particular, require thelowest possible power consumption for extended battery life and amaximal circuit integration to reduce device size and PC boardcomplexity. The quest for low-loss inductors is driven by a fundamentaltrade-off between power consumption on one hand and the need forlow-loss circuit passives (i.e., inductors and capacitors) on the other.Lowering the transistor bias in radio circuits reduces the powerdissipation, but also significantly degrades amplifier gains, oscillatorstability and filter selectivity. Using low-loss passives is the onlyviable technique to overcome this problem. Low-loss capacitors in the0.1 to 100 pF range are routinely integrated on chip nowadays. However,state-of-the-art integrated coil architectures are still too lossy to beof use in integrated RF designs. All present RF chipsets, therefore, arelimited to using discrete inductors that form a real estate bottleneckin today's increasingly miniaturized applications.

Modern wireless designs typically run in the lower GHz bands. Thestandard frequencies for cellular phones are 900 MHz, 1.8 GHz, 1.9 GHzand 2.4 GHz, while 900 MHz is the frequency of choice for digitalcordless phones. The 410-430 MHz, 870 MHz and 900-930 MHz bands are usedfor wireless RS-232, computer LANs and RF identification. At these 100MHz to GHz frequencies, the passives of choice are typically, forinductors, 1 to 30 nH and, for capacitors, 1 to 30 pF. The intermediatefrequencies in superheterodyne receivers are 40 to 350 MHz which callsfor passives in the order of 100 to 1000 nH and 10 to 100 pF. Althoughhigh quality on-chip capacitors ranging from 0.1 pF to 100 pF arecommonplace, integrated inductors and integrated variable capacitorsthat meet the low-loss requirements are currently not available.

Variable capacitors (varicaps) that can be integrated on chip are alsoin great demand. The benchmark architecture for contemporary wirelesstransceivers is still the superheterodyne architecture, which uses bothinductors and varicaps. Variable capacitors are essential components ofsuperheterodyne circuits used in many wireless devices. Superheterodynecircuits containing both inductors and capacitors currently cannot beintegrated on chip in commercial devices, and so present a bottleneck todevice miniaturization. The missing links in implementing fullsuperheterodyne wireless architectures on a chip are inductors withquality factors of at least 30 to 50, variable capacitors (varicaps)with a tuning range of 10% and quality factors of 30 to 50, andoscillators with quality factors of 10,000 or more. The processtechnology for making the capacitors should be compatible with theprocess for making the inductors.

Present wireless devices use discrete off-chip components to implementsuperheterodyne circuits. The very high Q oscillator is usually acrystal oscillator. There are also numerous Voltage ControlledOscillators (VCOs), each of which uses at least one discrete inductorand one discrete varicap. Because of these discrete components VCOsoccupy a large portion of many RF circuit area. Being able to integrateentire VCOs on chip requires a new type of varicap as well as inductor.

There is a need for a micromachined coil structure which is easy tomanufacture and does not use a lot of chip real estate. There is a needfor low loss coil structures and variable capacitors that can beintegrated on conductive substrates, such as silicon. There is also aneed for an integrated coil structure in which the windings have lowerresistance.

There is a need for a method of manufacturing a coil structure in whichclosing the turns of the coil electrically produces a viable electricalstructure. There is a need for a manufacturable technique that resultsin a closed coil structure suitable for high-Q integrated passiveinductor elements. There is a need for a manufacturing technique whichwould enable the integration of both on chip inductors and varicaps.

SUMMARY OF THE INVENTION

Accordingly, this invention provides a spring contact which exhibits thespeed and ease of solder-bump flip-chip bonding while eliminating theneed to create uniform solder bumps or uniform contacting pressure. Thisinvention further provides a spring contact which has elastic propertiesenabling the spring contact to maintain physical contact with a contactpad despite variations in contact pad heights, contacting pressure orthermal variations. This invention also provides an elastic springcontact having a stress gradient formed in the spring contact, whichcauses the spring contact to bend away from the substrate and thusprovide compliant contact with a contact pad. This invention furtherprovides a probe card and a method for producing the probe card havingspring contacts in place of standard probe needles.

The spring contacts of this invention are formed of a thin metal stripwhich is in part fixed to a substrate and electrically connected to acontact pad on the substrate. The free portion of the metal strip notfixed to the substrate bends up and away from the substrate. When thecontact pad on a device is brought into pressing contact with the freeportion of the metal strip, the free portion deforms and providescompliant contact with the contact pad. Since the metal strip iselectrically conductive or coated with a conductive material, thecontact pad on the substrate is electrically connected to the contactpad on the device via the spring contact.

Another embodiment of the invention overcomes the drawbacks of planarcoil structures in that the coil structures of the invention place thecoil axis parallel, rather than perpendicular, to the substrate plane. Acoil structure, according to the invention, includes a substrate and anelastic member having an intrinsic stress profile. The elastic memberincludes a first anchor portion fixed to the substrate, a loop windingand a second anchor portion connected to the substrate. The secondanchor portion and the loop winding are initially fixed to thesubstrate, but are released from the substrate to become separated fromthe substrate. An intrinsic stress profile in the elastic member biasesthe second anchor portion away from the substrate forming the loopwinding and causing the second anchor portion to contact the substrate.The resulting coil structure is out-of-the plane of the substrate. Theloop winding may also include a plurality of turns.

Various techniques may be used to position the second anchor portionaway from the takeoff point of the elastic member, either tangentiallyor axially. If the second anchor point is positioned tangentially fromthe takeoff point, the loop winding is generally in the shape of acircle, i.e., the second anchor portion contacts the substrate in thesame vertical plane as the first anchor portion. Various techniques maybe used to position the second anchor portion tangentially from thetakeoff point. For example, a mechanical stop can be fixed to thesubstrate at the desired location. Alternatively, the radius ofcurvature of the elastic member may be varied, such as by adding a loadlayer uniformly across the width of a portion of the elastic member orby patterning one or more openings or perforations uniformly across thewidth of a portion of the elastic member.

If the second anchor portion is positioned axially from the takeoffpoint or first anchor portion, the loop winding is generally in theshape of a helix. Several techniques may be used to form the loopwinding in a helix. For example, a uniform stress anisotropy may beintroduced into the elastic member, which causes a helical deformationin the released layer. Alternatively, the radius of curvature can bevaried in the elastic member to introduce a helical deformation. Thiscan be accomplished by varying the intrinsic stress profile in theelastic member as a function of position. A helical winding may also beformed by causing the resulting loop winding to have two (or more)different radii of curvature. This may be accomplished, for example, byforming one or more openings asymmetrically in the elastic member priorto release or by forming a load layer at an angle on a portion of theelastic member (upon release, the weight of the load layer causes theloop winding to be axially offset).

Various techniques can be used to connect the second anchor portion tothe substrate. For example, the second anchor portion can be soldered orplated to the substrate. Each anchor portion can be attached to a metalcontact pad attached to the substrate, for providing electricalconnectivity to other elements in a circuit. Preferably the elasticmember is formed of a conductive material. Alternatively, a layer of aconductive metal, such as gold or silver, may be plated on an innersurface, an outer surface, or both surfaces.

This novel structure allows, for the first time, the integration ofsubmillimeter-size high-Q inductors on both insulating and conductivesubstrates. When fabricated on a conductive substrate like silicon, thecoil structure produces much fewer magnetic flux lines that interceptthe substrate than present in-plane micro-coils, which then results infewer eddy currents induced in the substrate and lower coil losses.Furthermore, the coil structures may be used as inductors, which arecompatible with toroidal architectures that confine magnetic fieldsexceedingly well. This property enables multiple micro-coils to bepacked densely without coupling with each other. At high operatingfrequencies, skin and proximity effects increase the coil resistance.Unlike in-plane micro-coils, the out-of-plane coil structures can beeasily designed for low resistance operation without complicated highaspect-ratio processing. The out-of-plane coil structures are alsocompatible with numerous micro-coil embodiments such as center-tappedinductors and transformers for a wide range of applications.

A method for forming a coil structure, according to the invention,includes depositing a layer of an elastic material on a substrate, theelastic material having an intrinsic stress profile. The layer ofelastic material is then photolithographically patterned into an elasticmember. A portion of the substrate under the patterned structure isunder-cut etched to release a free portion of the elastic member fromthe substrate, an anchor portion of the elastic member remaining fixedto the substrate. The intrinsic stress profile in the elastic memberbiases the free portion of the elastic member away from the substrate,forming a loop winding and causing the free end to contact a point onthe substrate. This free end can then be connected to the substrate by,for example soldering or plating.

During the removal of sacrificial layers from the substrate, theintrinsic stress bends metal containing strips into the turns of aninductor coil. Fabrication of micro-coil structures requires controlover the coil geometry, in particular the coil radius, and, if a stressanisotropy is present, the helical pitch of the coil elements as well.If, for example, the loop has a constant radius r of curvature, and thelength of the released portion is 2πr, the free end will naturallyreturn to the take-off point of the loop. By placing a mechanical stopat a position away from the take off point, the free end can bepositioned and anchored. Magnetic structures can be created with suchloops by connecting the loops on the substrate with contact pads whichextend from the take off point of one loop to the contact point of anadjoining loop, producing a spiral. In another embodiment, the free endof the spring is offset axially and/or transversely with respect to thetakeoff point in order to provide for contact to adjacent loop pads.Mechanical and electrical contact is made permanent, for example, bysoldering, conductive adhesive, thermal compressive bonding or plating.

One aspect of the invention recognizes that it is possible to createhelical coiled structures of controlled diameter and pitch by exploitingstress anisotropy engineered into the deposited metal. The helicaltwisting provides the useful feature that the free end of the spring isshifted longitudinally (or axially) from the take-off point. Inprinciple, this allows formation of a continuous inductor consisting ofmultiple turns without interruption of the spring metal. It also allowssegments of more than one turn to be joined in order to produce aninductor. These structures reduce the number of coil-closinginterconnects, and thereby minimize the impact of interconnects oninductor quality factor.

Another aspect of this invention relates to producing a turn of a coilendowed with the property of non-constant radius. This allows the freeend of the elastic member to contact a point other than the take-offpoint of the loop, either tangentially from the takeoff point or offsetaxially from the takeoff point. Once a point away from the takeoff pointis contacted electrically, the un-lifted metal can be used to run atrace to any other point of an electrical circuit, including to anotherloop of an inductor. Several ways of varying the radius of curvature aredescribed, including varying the intrinsic stress profile along theelastic member, depositing a load layer along a portion of the elasticmember, and photolithographically patterning perforations in the elasticmember.

A new type of high-Q variable capacitor includes a substrate, a firstelectrically conductive layer fixed to the substrate, a dielectric layerfixed to a portion of the electrically conductive layer, and a secondelectrically conductive layer having an anchor portion and a freeportion. The anchor portion is fixed to the dielectric layer and thefree portion is initially fixed to the dielectric layer, but is releasedfrom the dielectric layer to become separated from the dielectric layer.An inherent stress profile in the second electrically conductive layerbiases the free portion away from the dielectric layer. When a biasvoltage is applied between the first electrically conductive layer andthe second electrically conductive layer, electrostatic forces in thefree portion bend the free portion towards the first electricallyconductive layer, thereby increasing the capacitance of the capacitor.

The manufacturing techniques of the invention can be used to create atunable LC combination employing a coil structure and variable capacitorto provide full superheterodyne wireless architectures on a siliconchip.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be described in relation to the following drawings,in which reference numerals refer to like elements and wherein:

FIG. 1 shows a chip wire bonded to a substrate;

FIG. 2 shows the chip tab bonded to the substrate;

FIG. 3 shows the chip solder-bump flip-chip bonded to the substrate;

FIG. 4 shows a probe card contacting an electronic device;

FIG. 5 shows a probe card having an angled probe needle;

FIG. 6 is a spring contact in an undeformed free state and anotherspring contact deformed when contacting a contact pad;

FIG. 7 shows a metal strip with no stress gradient;

FIG. 8 shows a model for determining the curvature of a spring contactdue to the stress gradient;

FIG. 9 shows a model for determining the amount of reaction forceexerted at the tip of the spring contact;

FIGS. 10-13 show the steps for one method of forming a spring contact;

FIG. 14 is a graphic representation of the film stress in a sputterdeposited nickel-zirconium alloy as a function of plasma gas pressure;

FIG. 15 is a top view of a spring contact;

FIG. 16 is a device for testing the contact resistance of a plurality ofspring contact pairs;

FIG. 17 is a graphical representation of the detected resistance of aplurality of spring contact pairs;

FIG. 18 is a graphic representation of the contact resistance of aspring contact as a function of the distance between the contact pad andthe substrate;

FIG. 19 is a spring contact having a flat end;

FIG. 20 is a spring contact having a pointed end;

FIG. 21 is a spring contact having two points t the tip end;

FIG. 22 is spring contact having multiple points at the tip end;

FIG. 23 is a spring contact having a deformable tab at the tip end;

FIG. 24 shows a spring contact having a deformed tab end when forcedagainst a contact pad;

FIG. 25 is a chip having a plurality of spring contacts electricallybonded to a substrate;

FIG. 26 is a chip bonded to a dust cover and having a plurality ofspring contacts electrically contacted to a substrate;

FIG. 27 is a chip bonded to a substrate and electrically contacted to asubstrate by a plurality of spring contacts on the chip having a dustcover;

FIG. 28 is a chip electrically bonded to a substrate by way of anintermediate wafer having a plurality of spring contacts;

FIG. 29 is a probe card having a plurality of spring contacts used fortesting an electronic device;

FIG. 30 is a liquid crystal display and a device for testing theoperation of the display;

FIGS. 31a and 31 b are cross-sections illustrating the currentdistribution at high frequencies in a disk shaped coil and a solenoid,respectively;

FIG. 32 is a cross-section of a stack of stress graded film depositedabove a release layer;

FIG. 33 illustrates a constant radius coil structure;

FIG. 34 is an SEM micrograph of a series of constant radius loops;

FIG. 35 illustrates a multi-turn coil formed from a series of connectedloops;

FIG. 36 illustrates positioning the second anchor portion with amechanical stop;

FIG. 37 illustrates positioning of the mechanical stop of FIG. 36;

FIG. 38 is a graph of tip trajectories for different elastic memberlengths for a coil radius of 100 μm;

FIG. 39 illustrates forming a multi-turn coil from individual tiltedcoils;

FIG. 40 illustrates a method of providing inter-coil connections;

FIG. 41 illustrates a toroidal solenoid;

FIG. 42 illustrates coil tapping;

FIG. 43 illustrates a simple air core transformer;

FIG. 44 illustrates an air-core transformer with intertwined primary andsecondary windings;

FIG. 45 illustrates an inductor with electroplated permalloy cores;

FIG. 46 illustrates laminating metallic cores;

FIGS. 47a and 47 b illustrate two stages of a micro-transformer;

FIG. 48 illustrates different helical pitch from varied springorientation;

FIG. 49 illustrates a multi-turn coil formed of single helical turncoils;

FIG. 50 illustrates a helically joined multi-turn loop;

FIG. 51 is a plot of a three segmented spring with three differentradii;

FIG. 52 illustrates a coil closed using a load member;

FIG. 53 illustrates transversely joined single turn loops;

FIGS. 54a and 54 b illustrate two structures having a varying radius ofcurvature;

FIG. 55 is a cross-section of a varicap according to the invention;

FIG. 56 is a graph of varicap capacitance versus spring lift; and

FIG. 57 is a top view of a varicap having a large array of individualcapacitor elements; and

FIG. 58 is a cross-section along line A—A of FIG. 57.

FIG. 59 is a tunable LC circuit.

FIG. 60 is a circuit diagram for the LC circuit of FIG. 59.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6 shows a side view of a bonding structure 100 having a pluralityof spring contacts 15. Each spring contact 15 comprises a free portion11 and an anchor portion 12 fixed to an underlayer or release layer 13and electrically connected to a contact pad 3. Each spring contact 15 ismade of an extremely elastic material, such as a chrome-molybdenum alloyor a nickel-zirconium alloy. Preferably, the spring contacts 15 areformed of an elastic conductive material, although they can be formed ofa non-conductive or semi-conductive material if they are coated orplated with a conductor material. More preferably, the spring contacts15 are formed of a nickel-zirconium alloy having 1% zirconium. Zirconiumis added to the alloy to improve the elastic properties of the alloywhile not greatly affecting the conductivity of the alloy. When theelastic material is not conductive, it is coated on at least one sidewith a conductive material, such as a metal or metal alloy.

The contact pad 3 is the terminal end of a communication line whichelectrically communicates with an electronic device formed on thesubstrate 14, such as a transistor, a display electrode, or otherelectrical device. The contact pad 3 is typically made of aluminum, butcan be made of any conductive material. The release layer 13 is made ofsilicon nitride, Si, Ti or other etchable material. However, the releaselayer 13 is not necessary and can be eliminated. The release layer 13and the contact pad 3 are formed on or over a substrate 14, which isformed of a material, such as oxidized silicon or glass or a printedcircuit board or ceramic or silicon or gallium arsenide.

As shown in FIG. 7, a strip of metal having no stress gradient inherentin the metal will lie flat. However, as shown in FIG. 8, when a uniformstress gradient is introduced into the strip of metal, the strip bendsinto an arc.

Each spring contact 15 is formed such that the stress gradient shown inFIG. 8 is introduced into the spring contact 15. When the spring contact15 is formed, the metal layer comprising the spring contact 15 isdeposited such that compressive stress is present in lower portions ofthe metal layer and tensile stress is present in upper portions of themetal layer. FIG. 8 shows the stress difference Δσ (i.e., the differencein stress from the top to the bottom) present in the spring contact 15.Compressive stress in lower portions of the metal layer is depicted byarrows directed to the left. Tensile stress is depicted in upperportions of the metal layer by arrows directed to the right. The stressgradient (stress difference divided by thickness) causes the springcontact 15 to bend into the shape of an arc having a radius r. Equation1 gives the radius of curvature r of the spring contact 15:$\begin{matrix}{r = {\left( \frac{y}{1 - v} \right)\frac{h}{\Delta \quad \sigma}}} & (1)\end{matrix}$

where y is the Young's modulus of the metal, h is the thickness of themetal layer forming the spring contact 15, Δσ is the stress difference,and ν is the shear modulus of the metal.

Referring again to FIG. 6, r is the radius of curvature of the freeportion 11 of the spring contact 15 as predicted in Equation 1, and θ isthe angle separating the radius line directed toward the junction of thefree portion 11 with the anchor portion 12 and the radius line directedtoward the tip 30 of the free portion 11. Equation 2 gives theapproximate height b of the spring contact tip 30 from the substrate 14for angles θ<50°: $\begin{matrix}{b \approx \frac{L^{2}}{2r}} & (2)\end{matrix}$

where L is the length of the free portion 11 and r is the radius ofcurvature of the free portion 11.

Since each spring contact 15 is preferably made of a highly elasticmaterial, each spring contact 15 can be pushed down at the tip 30 anddeformed as shown in FIG. 6, but will not plastically deform. Typically,a contact pad 3 of a device 101 exerts the downward force placed on thetip 30 and electrically contacts the tip 30. The spring contact 15resists the downward force placed on the tip 30 and maintains electricalcontact with the contact pad 3.

When the force on the tip 30 is released, the spring contact will returnto its undeformed state. Thus, the elasticity of the spring contacts 15allows the spring contacts 15 to make numerous electrical connectionswith different contact pads 3 while maintaining the integrity of theelectrical connection between the spring contact tip 30 and the contactpads 3.

Additionally, the spring contact 15 is preferably made of a creepresistant material. Therefore, when the spring contact 15 is elasticallydeformed over an extended period by a contact pad 3 pressing down on thespring contact tip 30, the spring contact 15 resists the downward forceand pushes the spring contact tip 30 against the contact pad 3,maintaining the electrical connection.

FIG. 9 shows a model for determining the amount of force F_(tip) appliedby the spring contact tip 30 to a contact pad 3 in reaction to the forceof the contact pad 3 pressing down on the spring contact tip 30.Equation 3 gives the reaction force F_(tip) of the spring contact tip30: $\begin{matrix}{F_{t\quad i\quad p} = \frac{w\quad h^{2}{\Delta\sigma}}{12x}} & (3)\end{matrix}$

where w is the width of the spring contact 15, h is the thickness of thespring contact 15, Δσ is the stress gradient and x is the horizontaldistance from the spring contact tip 30 to the point where the springcontact 15 first touches the substrate 14.

For a given width w, thickness h and stress gradient Δσ, the reactionforce F_(tip) of the tip 30 varies inversely with the distance x.Therefore, the reaction force F_(tip) increases as the spring contacttip 30 gets closer to the substrate 14, since the distance x decreasesas the spring contact 15 collapses and presses against the substrate 14as shown in FIG. 6. The increase in the reaction force F_(tip) as thecontact pad 3 presses the spring contact tip 30 closer to the substrate14 generally improves the electrical connection between the springcontact tip 30 and the contact pad 3. The increasing reaction forceF_(tip) causes the spring contact tip 30 to deform locally at thecontact pad 3, increasing the area of contact between the contact pad 3and the spring contact tip 30.

FIGS. 10-13 show the basic steps in one method for forming a springcontact 15. In FIG. 10, a contact pad 3 is formed on or over a substrate14. Additionally, an release layer 13 is formed on or over the substrate14. However, as mentioned above, the release layer 13 is not requiredand can be eliminated.

In FIG. 11, a layer of metal 16 is deposited on or over the substrate14. In the preferred embodiment of the invention, the metal is thenickel-zirconium alloy described above. Part of the metal layer 16 iselectrically connected to or directly contacts the contact pad 3 andanother portion of the metal layer 16 is deposited on or over therelease layer 13. There are many methods available for depositing ametal layer 16 on or over the substrate 14, including electron-beamdeposition, molecular beam epitaxy, chemical vapor deposition andsputter deposition. Preferably, the metal layer 16 is sputter deposited.

When sputter depositing a metal, the metal to be deposited is placed ona target and set to a high negative voltage. A stream of plasma gas,typically argon, is directed towards the target. The high voltagepotential between the plasma gas and the target metal produces ionswhich are accelerated toward and bombard the metal target. Thisbombardment knocks small particles of the metal target free and thefreed particles are guided to the surface where the particles aredeposited.

The metal layer 16 is deposited in several sub-layers 16-1 to 16-n to afinal thickness h of approximately 1 μm. The stress difference Δσ isintroduced into the metal layer 16 by altering the stress inherent ineach of the sub-layers 16-1 to 16-n of the metal layer 16, as shown inFIG. 11, each sub-layer 16-x having a different level of inherentstress.

Different stress levels can be introduced into each sub-layer 16-x ofthe deposited metal layer 16 during sputter deposition in a variety ofways, including adding a reactive gas to the plasma, depositing themetal at an angle, or varying the deposition angle, and changing thepressure of the plasma gas. Preferably, the different levels of stressare introduced into the metal layer 16 by varying the pressure of theplasma gas, which is preferably argon.

FIG. 14 is a graph showing the relationship of the film stress in thesputter deposited nickel-zirconium alloy and the pressure of the plasmagas used in the deposition. For low pressures of the plasma gas,approximately 1 mTorr, the film stress in the deposited metal iscompressive. As the pressure of the plasma gas increases, the filmstress in the deposited sub-layer changes to a tensile stress andincreases with increasing plasma gas pressure.

Preferably, the metal layer 16 is deposited in five sub-layers 16-1 to16-5. The first sub-layer 16-1 is deposited at a plasma gas pressure of1 mTorr, as indicated by numeral 1 in FIG. 14. The first sub-layer 16-1is the bottom-most layer in the metal layer 16 and has an inherentcompressive stress. The second sub-layer 16-2 is deposited on top of thefirst sub-layer 16-1 at a plasma gas pressure of approximately 6 mTorr.The second sub-layer 16-2 has a slight inherent tensile stress, asindicated by numeral 2 in FIG. 14. Sub-layers 16-3, 16-4 and 16-5 arethen deposited one on top of the other at the plasma gas pressuresindicated by numerals 3, 4 and 5 in FIG. 14.

The process of depositing the metal layer 16 in five separate sub-layers16-1 to 16-5 results in the metal layer 16 having a stress difference Δσwhich is compressive in the lower portion of the metal layer 16 andbecomes increasingly tensile toward the top of the metal layer 16.Although the stress gradient urges the metal layer 16 to bend into anarc, the metal layer 16 adheres to the release layer 13, the substrate14 and the contact pad 3 and thus lies flat.

After the metal layer 16 is deposited, the metal layer 16 isphotolithographically patterned into the spring contacts 15.Photolithographic patterning is a well-known technique and is routinelyused in the semiconductor chip industry. Photolithographicallypatterning the metal layer 16 is completed generally as shown in FIGS.11-13. A photosensitive material 17 is evenly deposited on the topsurface of the metal layer 16. The photosensitive layer 17 is thensoft-baked at a temperature of approximately 120° F. The photosensitivelayer 17 is then exposed to light, typically in the ultra-violetspectrum, using an appropriate mask. The mask ensures that areas of thephotosensitive material 17 are appropriately exposed to the light whichdescribes a two-dimensional view of the spring contacts 15.

Once the photosensitive material 17 is exposed to the appropriatepattern of light, the photosensitive material 17 is developed andhard-baked at a temperature of approximately 200° F. The elasticmaterial 16 is then etched to form the spring contacts 15. Differentmethods for etching can be used, including ion milling, reactive ionetching, plasma etching and wet chemical etching. Preferably, wetchemical etching is used.

The wet chemical etchant, for example, a nitric acid solution, isapplied to the elastic material 16. The etchant removes appropriateareas of the photosensitive material 17, determined by which areas ofthe photosensitive material 17 were exposed or not exposed to the lightand the type of photosensitive material 17 used. When the appropriateareas of photosensitive material 17 are removed, the etchant removes theareas of the metal layer 16 lying under the removed areas ofphotosensitive material 17. The remaining areas of the metal layer 16form the spring contacts 15. A top-view of one spring contact 15 isshown in FIG. 15. The area of the metal layer 16 removed by the etchantis described by the dashed line 18.

Next, as shown in FIG. 12, the free portion 11 of the spring contact 15is released from the release layer 13 by a process of under-cut etching.Until the free portion 11 is released from the release layer 13, thefree portion 11 adheres to the release layer 13 and the spring contact15 lies flat on the substrate 14. A second layer of the photosensitivematerial 17 is deposited on top of the spring contacts 15 and on thearea surrounding the spring contacts 15. The second layer of thephotosensitive material 17 is then exposed to light using theappropriate mask, developed and hard-baked. A selective etchant is thenapplied to the photosensitive material 17 and removes areas of thephotosensitive material 17 around the spring contacts 15. The etchant iscalled a selective etchant because after the areas of photosensitivematerial 17 around the spring contacts 15 are removed, the etchantproceeds to etch the release layer 13 underneath the spring contacts 15.The photosensitive material 17 on top of the spring contacts 15 resiststhe selective etchant and protects the spring contacts 15. The selectiveetchant etches the release layer 13 faster than the selective etchantremoves metal from the spring contacts 15. This means that the springcontacts 15 are released from the release layer 13 and are allowed tobend up and away from the release layer 13 due to the stress gradient inthe spring contacts 15.

Only those areas of the release layer 13 under the free portion 11 ofthe spring contact 15 are under-cut etched. The area of release layer 13under-cut etched for each spring contact 15 is described by the shadedportion in FIG. 17. This means that the anchor portion 12 of the springcontact 15 remains fixed to the release layer 13 and does not pull awayfrom the release layer 13. It should be appreciated that the method forpatterning the metal layer 16 onto the spring contacts 15 should notresult in any annealing of the metal layer 16.

Once the free portion 11 is freed from the release layer 13, the stressgradient causes the free portion 11 to bend up and away from thesubstrate 14. The stress gradient is still inherent in the anchorportion 12 and urges the anchor portion 12 to pull away from thesubstrate 14.

To decrease the chance of the anchor portion 12 pulling away from thesubstrate 14, the spring contact 15 can be annealed to relieve thestress in the anchor portion 12. This annealing process does not affectthe free portion 11 because, once the free portion 11 is released andallowed to bend up, no stress remains on the free portion 11 to berelieved by annealing. Thus, the stress gradient remains in the freeportion 11, and the free portion 11 remains curved up and away from thesubstrate 14 after annealing.

Finally, FIG. 13 shows a layer of gold 19 plated over the outer surfaceof each spring contact 15. The layer of gold 19 is preferably used toreduce the resistance in the spring contacts 15, but can be replacedwith any other conductive material. Preferably, the gold layer 19 isplated on the spring contacts 15 using a plating process.

Additional steps can be added to the under-cut etching process toimprove the process if necessary. For example, etchant vias, or smallwindows, can be etched into the free portions 11 of the spring contacts15. The etchant vias operate to provide the selective etchant fasteraccess to the release layer 13, thereby speeding the process ofreleasing the free portions 11 from the release layer 13. Also, a hardmask can be applied to the top surface of the spring contacts 15 toensure that the selective etchant does not remove material from the topsurface of the spring contacts 15 in case the photosensitive material 17protecting the top of the spring contacts 15 fails.

Since the process for forming the spring contacts 15 is limited only bythe design rules of photolithographic patterning, many hundreds orthousands of spring contacts 15 can be formed closely together in arelatively small area on the substrate 14. The typical width w of thespring contact 15 is 40-60 μm. Therefore, the spring contacts 15 can beformed close together, at a spacing of approximately 10-20 μm. Thismakes the center-to-center distance between adjacent spring contacts 15approximately 50-80 μm, which is well within the typicalcenter-to-center distance between adjacent contact pads 3 on a standardsemiconductor chip 2.

To test the effectiveness of the spring contacts 15 in applicationssimilar to those found in solder-bump flip-chip bonding, a test array ofthe spring contacts 15 at a center-to-center spacing of 80 mm wasdeveloped as shown in FIG. 16. Four sets of arrays 20 of the springcontacts 15 were formed on a bottom substrate 21. Four correspondingarrays of linked contact pads 22 were formed on an upper substrate 23.The upper substrate 23 and the lower substrate 21 were brought togethersuch that the spring contacts 15 contacted a corresponding contact pad3. The resistance R was then measured across pairs of the spring contact15 leads.

FIG. 17 graphically depicts the measured resistance R for each springcontact pair in the test apparatus. The measured resistance R withineach array generally trends upward from left to right because of theincreased conductor length of the spring contacts 15 positioned to theright compared to the spring contacts 15 positioned to the left in eacharray. Most of the approximately 25-30 ohms of resistance measured foreach spring contact 15 pair is due to the length and geometry of theconductors extending between the spring contacts 15 and the contact pads3.

FIG. 18 shows the total resistance of the connection between a springcontact 15 and corresponding contact pad 3. As shown in FIG. 18,approximately 1.5 ohms of resistance is due to the conductors leading tothe contact pad 3 and the spring contact 15. Approximately 0.2 ohms ofresistance is due to the shape of the spring contact tip 30. Theremaining resistance, approximately 0.5-0.8 ohms, is the resistance atthe interface between the contact pad 3 and the spring contact tip 30.

In general, the resistance at the interface between the contact pad 3and the spring contact tip 30 decreases as the height b decreases. Asmentioned above, the reaction force F_(tip) that the spring contact tip30 exerts against the contact pad 3 increases as the contact pad 3pushes the spring contact tip 30 closer to the substrate 14. Theincreased reaction force F_(tip) causes the spring contact tip 30 tolocally deform at the contact pad 3, thereby increasing the contact areaand decreasing the resistance at the interface.

The shape of the spring contact tip 30 takes different forms dependingon the application. FIGS. 19-24 show a series of six different tip 30shapes tested. Although only four of each type of spring contact tip 30were tested, none of the spring contact tip 30 shapes showed asignificant superiority over any other type of spring contact tip 30.

As mentioned above, since the production of the spring contacts 15 islimited only by the design rules of photolithographic patterning, thespring contacts 15 can be used to interconnect numerous different typesof devices. For example, FIGS. 25-26 show the substrate 14 having aplurality of spring contacts 15 formed on the top surface of thesubstrate 14. The contact pads 3 formed on the lower surface of the chip2 are electrically connected to corresponding spring contacts 15 on thesubstrate 14. An adhesive 24 holds the chip 2 stationary relative to thesubstrate 14. A dust cover, or can, 25 covers the chip 2 and ishermetically sealed to the substrate 14. The dust cover 25 assures thatmoisture and other foreign substances do not corrode the spring contacts15 or the contact pads 3, or otherwise interfere with the electricalconnections between the individual spring contacts 15 and thecorresponding contact pads 3.

FIG. 28 shows an alternate embodiment of a connecting device forelectrically connecting two devices. A wafer 26 is shown having aplurality of spring contacts 15 formed on opposite sides of the wafer.Pairs of the spring contacts 15 on opposite sides of the wafer 26communicate with each other and electrically connect the contact pads 3on both the chip 2 and the substrate 14. This embodiment of theinvention allows processing of the chip 2 and the substrate 14 withoutrisking damage to the spring contacts 15. The wafer 26 is used tointerconnect the chip 2 and the substrate 14 only after all processingis completed on the chip 2 and the substrate 14.

FIG. 27 shows another embodiment of the invention. The spring contacts15 are formed on the lower surface of the chip 2. The spring contacts 15contact corresponding contact pads 3 on the substrate 14. The adhesive24 holds the chip 2 stationary with respect to substrate 14.

The spring contacts 15 are not limited to interconnecting a chip 2 to asubstrate 14 or circuit board. The spring contacts 15 are used equallywell to interconnect two chips 2, two circuit boards, or otherelectronic devices to each other. One such alternative use for thespring contacts 15 is in probe cards. As discussed above, probe cards 7are used to temporarily connect two devices, typically when one of thedevices is tested. Such testing is common in the semiconductor industry,where the probe cards 7 are used to test semiconductor chips while thechips are still part of a single-crystal silicon wafer.

FIG. 29 shows an embodiment of the invention where the probe card 27 hasan array of spring contacts 15 used in place of the standard probeneedles 8. The probe card 27 operates identically to the standard probecard 7 except for having spring contacts 15. The probe card 27 isaligned with the device 10 such that the spring contacts 15 compliantlycontact the corresponding contact pads 3 on the device 10. The device 10is then tested or communicated with by a testing device electricallyconnected to the probe card 27.

An example testing device is shown in FIG. 30. A display patterngenerator 40 communicates with driver chips 42 mounted on the twofull-width probe cards 27. The probe cards 27 have the spring contacts15 which contact associated addressing lines 43 formed on the displayplate 44. The addressing lines 43 communicate with display electrodes(not shown). Therefore, the display pattern generator 40 can drive thedisplay electrodes to produce a matrix of electric potentialscorresponding to a test image. Sensors (not shown) on the sensor plate45 detect the matrix of electric potentials on the display electrodesand generate signals each corresponding to the electric potential. Thesignals are read out by scanner chips 46 mounted on the sensor plate 45.The test signal analyzer 41 receives the signals from the scanner chips46 and forms a sensed image corresponding to the signals. The testsignal analyzer 41 then compares the sensed image with the test imageoutput by the display pattern generator 40 to determine if the displayplate 44 and display electrodes are working properly.

Since producing a standard probe card 7 having probe needles 8 is laborintensive and time-consuming, standard probe cards 7 are not generallymade to contact all of the addressing lines 43 on the display plate 44.Therefore, testing of the display plate 44 must be done in sectionssince the probe cards 7 cannot accommodate the full width of theaddressing lines 43. In contrast, the probe card 27 made with springcontacts 15 can be made easily and inexpensively. Also, the probe cards27 having the spring contacts 15 can be made to any width and thereforecan test all of the data or address lines of an apparatus, such as thedisplay shown in FIG. 26, at one time.

The foregoing techniques for the manufacture of springs for probe cardsand other applications can be extended to the manufacture of coilstructures. Springs are made by introducing an intrinsic stress profileof a certain amount designed to produce the desired spring height andcurvature. Similarly, a reproducible built-in stress gradient orintrinsic stress profile can be designed into a thin film by varying thegrowth conditions appropriately during deposition to produce coilstructures, i.e., a spring which bends back on itself producing a loopwinding and contacting the substrate. By adding one or more conductivelayers, a coil structure suitable for use as an inductor may bemanufactured.

The intrinsic stress of many sputtered thin films depends on the ambientpressure at which the material is deposited. By varying the pressureduring sputtering, films can be obtained that are compressively stressednear the substrate-film interface and tensile stressed at the filmsurface. FIG. 32 shows such a stress-graded film 104 sandwiched betweentwo gold layers 102, 106. The stress graded film can be NiZr, Mo/Cr,solder-wettable Ni, or other suitable material. The bottom gold layer106 forms the outer skin of the coil when released and provides a highconductivity path for electrons at high frequencies. The top gold layerpassivates the surface. The metal stack is deposited above a suitablerelease layer 108 such as Ti, Si, or SiN. The release layer should be amaterial that can be quickly removed by selective dry or wet undercutetching. Possible etchants for a Si release layer include KOH (wetprocessing) and XeF₂ (dry processing).

In FIG. 33, a released structure with continuous layers 106 and 104 isshown. The challenge of connecting the free end of the loop to a contactpad on the same substrate is made difficult by the fact that the looptypically has a constant radius of curvature, and therefore the free endwill naturally return to the take-off point. Several techniques can beused to resolve this problem as described below.

The scanning electron micrograph in FIG. 34 shows a series ofout-of-plane micro-inductor windings produced according to theinvention. The coil windings were made using stress engineered thinfilms that are deposited by sputtering. The film isphotolithographically patterned into strips of micro-springs or elasticmembers that are subsequently released from their underlying substrate.Upon release, the built-in stress gradient causes the elastic members tocurl and form three-dimensional out-of-plane loops that make up theinductor coil. In the coil shown in FIG. 34, each loop has just enoughhelical pitch for each free end to contact the adjacent pad of thearray. The helical twisting provides the useful feature that the freeend of the elastic member is shifted longitudinally (or axially) fromthe take-off point. This allows for the formation of a continuousinductor consisting of multiple turns without interruption of the springmetal. To protect the inductor in actual use on a chip or circuit board,the loops can be enclosed in a molding compound.

In the specific example of FIG. 34, the stress graded metal is a 0.3μm-thick 85 Mo/15 Cr alloy deposited at five progressively increasingpressures. The film was patterned into 4 μm wide elastic members thatwere then released by removing an underlying PECVD SiN layer using 10:1buffered HF. The released elastic members formed 70 μm diameter circularloops. After rinsing in D.I. water, the released elastic members werepressed against a flat surface and the substrate was heated to 85° C.The compression holds the springs tightly while water is slowlyevaporated. This technique prevents liquid surface tension from pullingadjacent elastic members into a tangled mess as water evaporates. Formany applications, wider and thicker elastic members forming biggerloops are desired. These larger coils are easier to make than those inFIG. 34 because less stress gradient is required. Furthermore, widersprings are stiffer and generally less susceptible to getting entangledwith adjacent members during spring release.

FIG. 35 shows some of the process steps for forming a multi-turn coilstructure from a series of individual circular coil structures.Initially, a plurality of elastic members 61 a-65 a are patterned over arelease window. Each elastic member 61 a-65 a is part of a largerpatterned structure 61-65. For example, structure 61 includes elasticmember 61 a, connecting pad 61 b and contact pad 61 c. To form acontinuous structure, each loop must be electrically connected to thenext adjacent loop. After remove of release window 66, each elasticmember 61 a-65 a coils back on itself. When released, the elasticmembers 61 a-65 a will form circular loops with radiuses given byequation (1) above. The length of each of 61 a-65 a is designed so thatthe elastic members do not span complete loops when released. The tips(free ends) are left hanging just shy of an opposite contact, which isthe contact pad 62 c-65 c of the adjacent loop. The loops are thenpressed down on the contact and soldered or plated together. Theresultant multi-turn coil structure, with coil axis 68 begins at 61 a-61b, then the first loop winding 61 c, which is connected to contact pad62 a, and so on.

FIGS. 36 and 37 show an alternative approach for forming the coilconnections. In this approach, a mechanical barrier or stop 71 is fixedto the substrate at the end of contact pad 62 c, in order to receive thetip of elastic member 61 c. This approach uses almost full-lengthelastic members aided by a mechanical barrier 71. It is important todesign the dimensions of the mechanical stop properly and to positionthe stop correctly so that the structure lies entirely within thetrajectory of the tip as illustrated in FIG. 37. Otherwise, the elasticmembers may be caught on the near edge of the stop during release. InFIG. 37, the dashed line shows the tip trajectory.

FIG. 38 shows a graph of tip trajectories for 200 μm diameter coilshaving different elastic member lengths. The parameter in the figure, i,corresponds to length of the elastic member in multiples of πr/4, wherer is the coil radius. The x=0 point in the figure is the edge of therelease window. It is interesting to note that the tip of the fulllength spring, i=8, stays on the right of the release edge throughoutits entire trajectory. Since the mechanical block has to be placed atx<0, the length of the elastic member needs to be made less than a fullcircumference. The range of possible trajectories also place constraintson the dimensions of the mechanical block.

In addition to the mechanical stop, another method for positioning thefree tip tangentially, away from the takeoff point includes varying theradius of curvature of the elastic member. If the radius of curvature ifvaried along the length of the elastic member, a generally circular coilwill be formed. An unequal radius of curvature will cause the free tipto stop at some point away from the takeoff point. If the radius ofcurvature varies as a function of length and width of the elasticmember, a helical coil will be formed. The radius of curvature of theelastic member may be varied, for example, by adding a load layeruniformly across the width of one or more portions of the elasticmember. The radius of curvature may also be varied by patterning one ormore openings or perforations uniformly across the width of a portion ofthe elastic member. Some combination of load layer and perforations (oropenings) may also be used. Perforations and load layers may also beused to create helical windings as described below.

FIG. 39 shows another approach for forming a multi-turn coil withindividual loop windings. In this embodiment, release window 66 isdefined to have a skewed angle relative to the run length of eachelastic member 61 c-64 c. When the elastic member is released, the coilloops lean sideways contacting the adjacent contact pad. Thus loopwinding 61 c contacts pad 62 a. This lateral bending can also be inducedby designing a built-in stress anisotropy in the springs (describedbelow). When the springs are pressed down, their tips slide over toneighboring contact pads and a continuous coil is formed.

To take full advantage of the available conducting path, the coilthickness, h, should be made at least as thick as the skin depth δ:$\begin{matrix}{{h \geq \delta} = \sqrt{\frac{\rho}{{\pi\mu}\quad f}}} & (4)\end{matrix}$

where ρ is the resistivity of the coil conductor, μ is its magneticpermeability, and f is the operating frequency. Making the film thickerthan the skin depth does not improve film conductance because most ofthe current is confined to within the skin depth of the conductorsurface. For frequencies of interest (around 1 GHz), the ideal filmthickness is between 1 μm to 3 μm, a thickness range that is compatiblewith current deposition and patterning processes.

If the coil material is composed of one elastic material with a stressgradient, with the film thickness determined, the coil loop radius canbe calculated using equation 1. If there are additional layers, thestress profile is not a linear gradient and equation 1 needs to bemodified. The spring length, l, should then be designed to about

l=2πr  (5)

for the elastic members to form complete circular loops when released.The number of coil turns, N, is next determined based on a desiredinductance which approximately equals: $\begin{matrix}{L = {\mu_{0}\frac{N\quad \pi \quad r^{2}}{x}}} & (6)\end{matrix}$

where x is the pitch between coil windings and μ₀ is the permeability ofair (for air-cored coils). While equation 6 is good for toroids and longsolenoid (N*×>>r), more complicated expressions for short solenoids areavailable from textbooks. The spring width, w, can be made as wide asnecessary to accommodate an acceptable electrical resistance, R, throughthe following approximation: $\begin{matrix}{w = \frac{2\pi \quad r\quad \rho \quad N}{\delta \quad R}} & (7)\end{matrix}$

Equations 6 and 7 indicate that there is a tradeoff between inductanceand resistance. Wide elastic members, small number of loops, and a shortradius produce low resistance but also lead to low inductance. The ratiobetween coil impedance and resistance, also called the coil qualityfactor Q, is a good parameter for assessing how losses influence coilperformance: $\begin{matrix}{Q = \frac{2\pi \quad {fL}}{R}} & (8)\end{matrix}$

This dimensionless parameter determines the sharpness of the resonancepeaks of LC resonators, the selectivity of LC filters, the amount ofoscillator jitter, and the gain of resonant amplifiers. Looking nowagain at equations 6 and 7, it can be seen that the quality factorincreases with coil diameter and with the ratio between conductor widthto the winding pitch: $\begin{matrix}{Q \approx {\frac{\pi \quad f\quad \mu_{0}}{\rho/\delta}\frac{w}{x}r}} & (9)\end{matrix}$

Also the importance of a low AC sheet resistance, ρ/δ, is explicitlyexpressed in equation 9.

Table 1 tabulates a few representative inductance values and Q factorsfor out-of-plane coils produced in accordance with the invention. Aconductor resistivity of 2.5 μΩQ-cm is assumed in the estimates. Thequality factors are roughly approximated by supposing that the currentflows as a uniform sheet with a sheet thickness equal to the skin depth.The actual quality factor may be up to a factor of 2 smaller due toproximity effect that is not included in these calculations. The listedQ numbers should be compared to the best values of 10 to 20 that arecurrently obtained with state-of-the-art in-plane coils utilizing highaspect ratio windings and removed substrate.

TABLE 1 Matrix of Values for Typical Air-Cored Out-of-Plane InductorCoils Coil Coil Line width Line pitch diameter length L Q @ [μm] [μm][μm] [μm] # turns [nH] 1 GHz 4 8 70 76 10 6.4 7.3 4 8 70 796 100 61 7.024 30 200 294 10 13 32 24 30 200 2994 100 132 32 54 60 500 594 10 41 9054 60 500 5994 100 412 90 90 100 1000 990 10 100 180 90 100 1000 9990100 988 180

In addition to the “diagonal” release window for connecting individualloop windings to one another as described in FIG. 35, many other typesof connections are possible. An alternate embodiment is shown in FIG. 40which utilizes symmetric wedge take-off points. In FIG. 40, elasticmembers 81 a-85 a are deposited and patterned on a substrate. Eachelastic member, for example, 81 a, includes a patterned contact padarrangement. This contact pad arrangement includes a U-shaped portion 81b, which includes two tip portions, 81 c and 81 d. Also included forsupport is symmetric element 81 e. The symmetric supports balanceopposing biaxial stresses in the released film 81 a in order to reducelateral bending of the coil windings. Placing the release point lowerthan a mating contact pad also brings the elastic member tip to theproper contact point without mechanical blocks. This design alternativeenables better contacting at the expense of a slightly longer windingpitch. When elastic members 81 a-85 a are released, they coil andcontact pad portions 82 c-86 c(not shown).

The multi-turn coil designs in FIGS. 35, 36, 39 and 40 provide linearcoil arrangements, i.e., the coil axis is a straight line. Each of thesedesigns can also be arranged in a circular layout to form micro-toroids,i.e., the coil axis is in a circle. A micro-spring toroid is shown inFIG. 41, with coil axis 91 and each coil turn 92 shown schematically.Toroids are attractive because they confine magnetic fields very tightlywithin their windings, thus allowing multiple coils to be packed veryclosely without mutual coupling. The absence of stray magnetic fieldsalso further reduces lossy substrate eddy currents.

Unlike in-plane coils, the individual windings of out-of-plane coils areeasily accessible at arbitrary locations along the inductor. Therefore,it is possible to obtain different inductances from a single coil bytapping the windings at appropriate locations. When combined withtransistor switches, these taps can be used to make variable inductorsuseful in tunable filters and resonators. FIG. 42 shows how the coil ofFIG. 41 can be modified by adding tap 93 at contact pad 61 a, tap 94 atcontact pad 62 a and tap 95 at contact pad 65 a. Note that the tappoints depend on N, the number of windings between taps. Between taps 93and 94, N=1 and between taps 93 and 95, N=4.

In addition to their use as inductors, the out-of-plane coils can beused as transformers. Micro-transformers are essential in electroniccomponents such as mixers, double-tuned filters and RF signaltransformers. The out-of-plane coils are compatible with a variety ofmicro-transformer architectures. FIG. 43 shows an embodiment in the formof a toroidal transformer with an air core, which includes primarywinding 124 having input/output 122 and secondary winding 126 withinput/output 120. The voltage relationship between the two coupled coilsis determined by the ratio of turns between the primary and secondarywindings. The pairs of arrows 120 and 122 indicate current paths intoand out of the two windings, 124 and 126.

FIG. 44 shows an alternate design for an air-core transformer withintertwined primary 124 and secondary 126 windings. The multipleoutbound arrows 127 in the secondary coil 126 illustrate the possibilityof coil tapping for obtaining variable transformer ratios. The insetshows the micro-spring layout necessary for implementing the transformerarchitecture. Coil tapping is, naturally, also compatible with thedevice in FIG. 43.

Ferromagnetic cores are attractive for many coil applications because oftheir ability to increase coil inductance and to channel and confinemagnetic fields to well defined regions. For high frequency GHzapplications, however, any ferromagnetic material used has to beelectrically insulating. Otherwise, excessive loss leading to low Q willresult.

The micro-coils can be embedded in an epoxy matrix that contains ferriteparticles, after they are released from the substrate. This creates aferrite core in and around the micro-coil that increases the coilinductance. It is also the method of choice to confine the magneticfields of solenoids. The field lines outside the solenoid do not fan outanymore because the ferrite around the coil closes the magnetic pathway.

Coils are magnetically isolated from each other by using an island offerromagnetic material for each individual coil. Coil windings aretherefore placed in deep pockets made by patterning spin-coated BCB oranother thick film. After release of the elastic member, the pocket isfilled with ferromagnetic particles of a suitable size immersed in aninsulating epoxy matrix.

Another approach utilizes ferromagnetic metal cores that can bedeposited and patterned in ways that are compatible withmicrofabrication. However, since these cores are conductive, theirapplications are limited to lower frequencies. FIG. 52 shows such adevice employing an electroplated permalloy (NiFe) core. In thisembodiment, a layer of SiN_(x) 202 is deposited on the substrate 200,followed by the elastic member 204. A thick film 206, such as SU-8photoresist, is first patterned to define a window for plating the corematerial. The NiFe core 208 is plated above a thin vacuum-deposited seedlayer which in turn lies on top of an insulating dielectric 210. TheSU-8 layer 206 is then removed, followed by release of elastic member204 to form loops that enclose the core.

The coil losses can be reduced to some extend by laminating the core 208as shown in FIG. 52.

FIGS. 45 and 46 are not drawn using actual scale and aspect ratios. Inparticular, the core 208 has to be designed so that it conforms to theconstraints discussed in FIG. 38. This constraint makes the core occupymuch less than the available cross sectional area of the coil. However,for typical core relative permeability of about 1000, even a 10% fillfactor will increase the inductance of an air-core device by about100-fold.

A metallic or ceramic ferromagnetic core can also be formed byphysically attaching a pre-made core 206 on the substrate 200 prior torelease of the elastic members 220 as shown in FIG. 47. The placementcan be performed by an automatic pick and place equipment commonly usedin the chip industry. The dimensions of the pre-made core will,naturally, also have to conform to the same constrains discussed in FIG.38.

FIG. 47 illustrates how a ferromagnetic core micro-transformer can befabricated using the methods described above. FIG. 47a shows the elasticmember 220 layout prior to release. Two sets of metal lines facingopposite each other for the primary and secondary windings are placedwithin the BCB pocket. After release of the elastic members 220, thepocket is filled with a ferromagnetic epoxy. An illustration of thereleased elastic members is shown in FIG. 47b. The loop 224 in FIG. 47btraces a magnetic path coupling the primary and secondary coils. Thepocket is designed with features that extend towards the coil axismidway down each coil. These features are meant to obstruct stray fieldsand improve the intended coupling between the primary and secondarywindings. Although the estimated coupling for the transformer in FIG. 47is only about 66%, significant improvements are possible ifphoto-definable filling materials are used.

An alternative ferromagnetic core micro-transformer can be fabricatedfrom the methods discussed in FIGS. 45 and 46. In this embodiment, thecore of FIGS. 51 and 52 is fashioned in a loop that magnetically couplesa set of two or more coil windings. To reduce the possibility of coresaturation, a small air gap can be placed to break the core loop.

The above described coil structures have circular loop windings. Suchcoil structures can also be manufactured using coils with a helicaltwist.

It has been observed that a helical twist develops in some releasedstructures. The origin of this twist is stress anisotropy. Specifically,in a planetary deposition system, the radial and tangential componentsof the stress in the film vary at different rates, producing stresses ofdiffering magnitude. The stress anisotropy gives rise to aradial-tangential shear. The pressure in the sputter system is variedduring deposition to produce a stress gradient, however, because thestress is anisotropic, a shear gradient develops as well. This applies atorque to the spring, giving it a finite helical pitch. The helicalpitch causes the tip of the released spring to move off the axis of thespring.

It has also been observed that wider finger structures tend to lift morethan narrow finger structures of the same thickness. Springs can twistin only one direction at a time, and so cannot relax stress completelyin one direction. As plane strain conditions exist near the longitudinalcenterline of wider springs, intrinsic longitudinal stresses relaxcompletely, while transverse stresses can only relax very near theedges.

FIG. 48 illustrates how different helical pitch results from variedspring orientation. The springs were made from metal deposited in aplanetary sputter system. The planetary motion of the wafer in thevacuum system produces geometric differences in the flux arrival in theradial and tangential directions of the wafer. This causes the stress inthe radial and tangential directions of the wafer to be unequal. Twoloops are shown, the one to the left 130 is oriented along a directionof principle stress, and as a result, the helical bending is practicallyzero. The spring on the right 132 in FIG. 48 is oriented at 45 degreesto the principle axis, and as a result has a large helical pitch, on theorder of the loop diameter. Therefore, by taking a metal film of knownstress anisotropy, in this case about 8.6%, and by orienting the springat the desired angle to the principle axis, the pitch may be usefullycontrolled.

Inventor David Fork's U.S. application Ser. No. 09/572,200, which wasfiled on May 17, 2000, and is incorporated herein by reference,discloses manufacturing methods for sputtering thin films withcontrolled stress anisotropy. Other methods for creating a helical twistin the loop winding are disclosed below.

FIG. 49 illustrates how a multi-turn coil 140 utilizing single helicalturns 142 might be configured on a substrate. Each turn of the coil hasa sufficient degree of helical pitch to jog the free end of the loopover to a contact pad 144 adjacent to the loop 142. The free end inmaking mechanical contact with the pad may also make electrical contact.Robust electrical and mechanical contact can be improved by for examplesoldering the free end of the loop to the pad 143. The illustration inFIG. 49 shows a jog between the end of the first loop and the base ofthe adjacent loop. This is done for clarity, and is not necessary forthe actual device. The performance is better for coils with denserwindings, therefore it is advantageous to pack the coils as tightly aspossible.

FIG. 50 shows a multi-turn coil 150. For such a coil, the spring metalis patterned into a strip that is long enough to span the entire coil.The illustration shows 4 turns. In principle, the number of turns islimited by the length of the substrate, since the length of the springis given by the product of the number of turns and the loopcircumference. If it is not practical to make a single inductor out of asingle multi-turn coil, multi-turn segments could be joined using thepad contact points shown in FIG. 34 to produce a complete device.

One possibility with a multi-turn loop segment of FIG. 50 is to producemore densely wound coils. For coils produced from loops of single turns,layout considerations limit the loop spacing to be at least slightlygreater than the width of the spring metal in the loop. Multi-turn loopshowever do not have this restriction, because the springs are madelonger in the transverse coil direction to accommodate the multipleturns. A long strip of metal can coil with a helical pitch less than itswidth, and the free end can overlap a contact pad either via thecumulative offset of the helical pitch, or by a tab the extends from thefree end of the spring to the pad. To prevent shorting of theoverlapping turns of the multi-turn loop, one surface, preferably, thetop surface of the spring metal can be covered or partially covered withan insulating spacer layer. This technique may require tighter controlof the radius and pitch since error in free end placement wouldaccumulate with each turn.

Other methods can be used to displace the free end of the elastic memberwith respect to the takeoff point. Varying the radius of curvature ofthe coil will displace the free end in a transverse direction. Theradius of curvature depends on the amount of intrinsic stress profile inthe elastic member and on the mechanical properties of the elasticmember. To obtain a desired twist, an elastic member can be formed withan intrinsic stress profile of one value in a first portion and a secondvalue of intrinsic stress profile in the remaining portion. Anothermethod is to put in anisotropic properties by, for example, depositing aload layer on one side of the elastic member. When the elastic member isreleased, the resulting coil will have two sections, each with adifferent radius of curvature. The effect of the two different radii ofcurvature is that it forces the elastic member to twist.

While a coil with sections having two different radii of curvature canbe employed to offset the landing position of the tip from the takeoffpoint of the released elastic member, a preferred configuration is onewith three sections of different radii of curvature. FIG. 51 shows aplot of a roughly 0.5 mm diameter loop wherein the elastic member wasdesigned to contact tangentially to a point roughly 150 microns behindthe take-off point of the elastic member. The top half of the coil iscomposed of a larger radius than the bottom half. This has the effect ofdisplacing the free end backward with respect to the take-off point.Second, by making the bottom (first and fourth) quarters (first andthird segments) of the spring have a smaller radius than the topsegment, the free end of the spring contacts the substrate tangentially.Tangential contact may be advantageous for increasing the area ofcontact, and thereby lowering the contact resistance. Tangential contactmay also reduce the sensitivity to placement errors. Note that the radiifor the first and third segments are equal, there is no need to createmore than two different radii; this simplifies processing.

Another way to vary the radius of curvature is by incorporating a loadlayer on either the inner surface of the elastic member or the outersurface (or both). The load layer is an additional layer patterned onthe elastic member to apply stress that either increases or decreasesthe bending radius. The bending radius, R, for a loaded beam can beexpressed as $\begin{matrix}{R = \frac{{Y_{o}^{2}h^{4}} + {2Y_{o}Y_{1}h\quad {t\left( {{2h^{2}} + {3h\quad t} + {2t^{2}}} \right)}} + {Y_{1}^{2}t^{4}}}{{{\Delta\sigma}\quad {h^{2}\left( {{h\quad Y_{o}} + {t\quad Y_{1}}} \right)}} + {6\left( {{\sigma_{1}Y_{o}} - {\sigma_{0}Y_{1}}} \right)h\quad {t\left( {t + h} \right)}}}} & (10)\end{matrix}$

where Y₀ is the spring modulus, Y₁ is the load layer modulus, h is thespring thickness, t is the load layer thickness, Δσ is the springintrinsic stress variation, σ₀ is the net intrinsic stress in theelastic member, and σ₁ is the load layer intrinsic stress.

In the example in FIG. 51, the two radii for the first and secondsegments could be produced with the following parameters:

Elastic Member Nickel alloy Member Stress Gradient 1 GPa Member NetStress 0 GPa Member Thickness 970 nm Load Metal Gold Load Stress 0 GPaLoad Thickness 180 nm

The load layer is patterned to reside only on the middle segment of theelastic member. Note that the equation 10 assumes purely elasticbehavior, and may be approximate. Gold may relieve some of its stress byplastic flow. This may modify the thickness required somewhat. Othermaterials, with higher yield points can be substituted for gold as theload materials.

FIG. 52 shows a coil produced with a tangential offset by incorporationof a load layer. The structure of FIG. 52 may be produced in accordancewith the following process. First, a release layer 301 of 100 nm Ti isdeposited on a substrate (not shown).

Next the outer coil conductance layer 302 (which is preferably gold, butmay be any other suitable conductor) is deposited. Then the elasticmember material 303, which is NiZr, is deposited on the conductor layer302. A load layer 304, which is preferably a metal layer of gold, isthen deposited on the elastic member. The locations of the solder padsare then masked with photoresist, followed by plating of solder onto thesolder pad areas.

The solder pad mask is then stripped and the load layer is then maskedwith photoresist. This provides the location of the load layer. The loadlayer is then etched with potassium iodide and the load layer mask isstripped. Next the elastic member is masked with photoresist. Theelastic member 303 is then etched with nitric acid to form theunreleased coil. Then the coil conductance layer 302 is etched withpotassium iodide. To clear between the elastic members, the releaselayer 301 is etched, preferably by dry etching in a fluorine plasma. Theelastic member mask is stripped and then the release window is maskedwith photoresist. The release layer is removed through the releasewindow using hydrofluoric acid. If desired, the release window mask canbe stripped. When the release layer is removed, the intrinsic stressprofile in the elastic member 302 causes the elastic member to coil onitself. The load layer 304 causes a tangential offset, which enablescontact with a contact pad. Flux is applied to the solder contacts, thesolder reflows. Preferably an epoxy is applied over the resulting coiland cured. Finally the wafer is diced.

The resulting coil structure in FIG. 52 illustrates that it is possibleto create a useful coil-closing structure with as few as two segments.FIG. 53 illustrates a top view of a completed transversely joined singleturn loops.

The radius of curvature of the coil segment can be varied by placing aload layer asymmetrically across a segment of the elastic member or byintroducing one or more openings asymmetrically in the elastic memberprior to release. We have observed a size effect for the bending of thespring, which arises because the edges of the spring are able to relaxsome of the intrinsic stress. Narrower springs relax more of the totalstress at their edges than wide springs. For springs of varied width, orslotted springs, a theory has been worked out. Essentially, theeffective biaxial modulus of the spring can be varied between the limitsdefined by Y/(1−ν) and Y/(1−ν²), where Y and ν are the Young's modulusand Poisson ratio respectively. For typical values of ν the radius canbe varied by about 30% by slotting the spring, or varying its width. Asimilar effect is possible by placing holes (openings 162 in elasticmember 160 as shown in FIG. 54a) rather than slots (slot 172 in elasticmember 170 as shown in FIG. 54b) into the elastic member; this wouldproduce two dimensional stress relaxation. This effect can be exploitedby perforating the top segment of the elastic member in order to make itbend to a larger radius. For practical reasons, it is better to slot theelastic member into as few strips as needed in the top segment in orderto maximize the conductance.

Advantages of perforation are that it removes the need to separatelydeposit, mask, and pattern an additional layer, such as a load layer.The process is therefore less expensive. A further advantage is that italleviates the need to control the materials properties of the loadlayer, thus simplifying the process, and increasing yield. The examplespring shown in FIG. 51 could have been created by slotting the middlesegment of a MoCr spring with a thickness of 1.75 microns and anintrinsic stress profile of 2.8 GPa.

A further application of perforation is to produce a controlled helicalpitch, not by growing in an intrinsic stress anisotropy as describedabove, but by instead slotting the elastic member to produce a nettorque. A slot 172 running down the length of a segment of the elasticmember 170, and offset to one side, will cause the two sides of thesegment to bend to different radii. This will pull the segment into ahelix. Other asymmetric configurations may also have utility, such asdiagonal slots or load layers, or off-center holes or load layers. Avariable radius coil would also allow higher fill factors for NiFe coresby relaxing the constraints of FIG. 38.

A significant challenge to making a useful coil is making the coilresistance low (high Q factor). An aspect of the micro-coils describedabove is that high Q inductors may be created by adjusting the springwidth, and outer conductor resistivity, and the outer conductorthickness. Because the skin effect confines the current to the outersurface of the coil, these factors dominate the high frequencyresistance of the inductor loop.

The resistance of the loop closure may also be limited by connecting thefree end of a loop back to a contact pad on the substrate with lowresistance. Obtaining low resistance at the contact pad requires a goodmetallurgical junction consisting of highly conducting materials. Belowwe describe a structure and manufacturing embodiment that achievesmetallurgical junctions with low contact resistance. Coil structuresincorporating a solder pad that is reflowed to close the loop has beendescribed above and achieves a good metallurgical junction as well lowcontact resistance. Alternatively, the free end may be joined to thecontact pad by plating, either electroless or electroplating. In thismethod, the loop is formed by releasing the elastic member. The free endcomes into either mechanical contact or proximity to a contact pad onthe inductor substrate. Then, plating applies conducting material aroundboth the free end and the contact pad, forming a continuous jointbetween them. In this embodiment, the application of material need notbe limited to the free and the pad areas only. Preferably, the platedmaterial has high conductivity, and is plated throughout the loop inorder to reduce the coil resistance, thereby beneficially increasing thequality factor.

The method of the invention permits process extensions. These processflows are exemplary, but other variations are possible. For example,certain process steps described above with respect to FIG. 52 may becombined or eliminated. Layers of solder used to close the loop, couldalso serve as the release window for the spring release step.

The foregoing techniques can also be used to manufacture a new type ofhigh-Q varicaps. These varicaps use the same microspring technologydescribed above, have the requisite capacitance values, and can beintegrated on chip. A varicap structure based on micro-springs allowsboth missing on-chip RF passive components, inductors and varicaps, tobe fabricated using the same process technology. These micro-springvaricaps have the additional benefit of requiring lower bias voltagesthan parallel plate MEMS capacitors. By using a spring as the secondelectrode in a photolithographically patterned capacitor, and varyingthe voltage between a fixed plate and the spring, the capacitance of thestructure varies.

FIG. 55 shows a cross-section of a variable capacitor employing themicro-spring technology. A layer of metal 153 (metal 0) is firstdeposited and patterned to the desired shape on a substrate (not shown).Next a layer of a dielectric material 156 is deposited and patternedover the metal layer 153. Over the dielectric layer 156, a release layer152 is deposited. Then metal layer 151 (metal 1) is deposited over therelease layer 152. Metal layer 151 is an elastic material with aninherent stress profile built in. This inherent stress profile is builtinto the layer in the same manner as described above with respect to themicro-springs. Metal layer 151 is patterned to the desired spring shape.When the release layer 152 is patterned and partially removed, theinherent stress profile in the metal layer 151 biases the free portionof metal layer 151 away from dielectric layer 156 covering the metallayer. If an insulating material is used for the release layer 152, thedielectric layer 156 may not be necessary.

The capacitance is defined by a suspended undercut section of length L₁in parallel with a fixed portion of length L₀. If a DC bias is appliedbetween layer 153 and layer 151, electrostatic forces will cause thesuspended part to bend down and increase the AC capacitance.

FIG. 56 plots the capacitance as a function of the spring lift, d, forthe specific case of L₀=25 μm, L₁=100 μm, d₀=0.5 μm, capacitor width=500μm, and r=500 μm. In a VCO circuit, the spring radius of curvature, r,could be designed so that it is identical to the loop radius of theaccompanying inductor. This way both inductors and varicaps can befabricated in the same step.

FIG. 56 shows that the varicap capacitance changes from 2 pF to 2.2 pFwhen the tip is deflected from 10 μm to 7 μm. This 10% tuning rangecorresponds to a deflection that is well below ⅔ of the initial lift, sothere is no danger of bi-stable operation where the spring suddenlysnaps down. The estimated voltage required to deflect the cantilever by3 μm is only about 10V. This low voltage is due to the curved electrodeprofile, which generally requires lower drive voltages than moreconventional actuators. For larger deflections, one can considertapering the spring tip to delay the onset of bi-stable behavior.Alternatively, one can make a tapered electrode (layer 151 in FIG. 55)under a conventional spring.

Varicaps made according to the above processes exhibit excellentimmunity to vibration. The curved electrode profile allows thecantilever to be made stiffer than in parallel plate devices resultingin devices with low sensitivity to inertial forces. Under acceleration,the ratio of inertial forces to electrostatic forces is only in theorder 10⁵.

An array of variable capacitors can be arranged into a single device toproduce a larger capacitance. FIG. 57 shows an example of a largervariable capacitor. Referring to FIGS. 57 and detail FIG. 58, a largebottom conductor layer 268 is deposited on a substrate 269. Contact 266provides the contact for the bottom electrode, which may be multipleelectrodes connected electrically together or a single bottom conductorlayer. A dielectric layer 267 is deposited on top of conductor 269followed by a release layer 270. On top of the release layer 270 isdeposited the second conductor layer 261, which is patterned into theconfiguration of parallel rows of “springs” 261, each connectedelectrically by a bus connector 263. The height of the micro-springs261, determines the capacitance, and is controlled by applying a voltagebetween the springs contact 264 and the contact for the bottomelectrodes 266. In some embodiments, if the release layer 270 is formedof an electrically insulating material, the portion of the release layerremaining beneath the first conducting layer 261 functions as thedielectric layer. This eliminates the need for depositing a separatedielectric layer. However, in most applications, it is preferred to havethe dielectric layer 267 extend completely between the first and secondconducting layers to prevent shorting.

The method of the invention can be easily applied for on-chip circuitapplications requiring an LC circuit or a tunable LC circuit. Referringto FIGS. 59 and 60, a tunable LC circuit is shown. The microcoil 270connects a tunable capacitor 272 formed by plates 284 (A), and 282 (B),with common dielectric layer 286 (D). Applying a DC bias between plates284 (A) and 282 (B) controls the value of the capacitance. A DC blockingcapacitor formed by plates 280 (C) and 282 (B) prevents the microcoilfrom shorting the bias source. Note how the microcoil 270 attaches tothe DC blocking capacitor at point 290. The capacitor top plates 284 (A)and 280 (C) are implemented preferably using the same metal as for themicrocoil 270. The bottom plate 282 (B) is made of an additional metallayer.

Processing is achieved economically. First the bottom conductor layer D(286) is deposited on the substrate and etched. Then the dielectriclayer 286 is deposited followed by a single release layer (not shown)which covers the area of both capacitor BC and microcoil 270. A metallayer C is deposited. Then a metal layer formed of an elastic materialfor both capacitor layer A and microcoil 270 is deposited and shaped.When the release layer is undercut, the microcoil and variable plate Aare formed. The free ends of the microcoil are attached using one of themethods describe above.

Example: Varicap AB with a tuning range of 500 μm by 550 μm variablecapacitor, 500 nm Si₃N₄ dielectric (∈_(r)=8)=3.5 to 22.7 pF with aminimum overlap=500 μm by 50 μm, maximum overlap at the snap-downlimit=500 μm by 320 μm. At this point the tip of plate A is down by66%.). Blocking capacitor DC of size 400 μm by 1.6 mm, 500 nm Si₃N₄dielectric layer (∈_(r)=8)=91 pF. The tuning range of both capacitors inseries=3.37 to 18.2 pF. The Micro-solenoid 270 has a 1 mm diameter, 5windings, 500 μm long=26 nH. As a result, the tuning range of the LCresonance frequency=538 to 232 MHz.

The invention provides a new type of high Q micro-inductors that can beintegrated on Silicon ICs. Unlike most previous micro-coils, the coilstructures feature an out-of-plane architecture where the coil axis isplaced parallel to the wafer surface. The out-of-plane coils address theproblem of induced substrate eddy currents associated with in-planeinductors. It also provides a simple way to counter the increasedelectrical resistance caused by skin effects without resorting to highaspect ratio processing. The design is compatible with a large varietyof related embodiments such as coil tapping and transformers. Thisinvention supplies a major missing element in integrated RF circuitdesign.

A new type of high Q micro-spring variable capacitors and out-of-planeinductors that can be integrated on Silicon ICs has been described.These varicaps when combined with inductors can be implemented foron-chip integration of entire VCOs in superheterodyne circuits. Whilethe invention has been described with reference to specific embodiments,the description of the specific embodiments is illustrative only and isnot to be construed as limiting the scope of the invention. Variousother modifications and changes may occur to those skilled in the artwithout departing from the spirit and scope of the invention.

What is claimed is:
 1. A circuit structure, comprising: a substrate; acapacitor comprising a first electrically conductive layer fixed to thesubstrate and a second electrically conductive layer comprising ananchor portion and a free portion, the anchor portion being fixed to thesubstrate and electrically insulated from the first electricallyconductive layer, wherein a stress profile in the second electricallyconductive layer biases the free portion away from the firstelectrically conductive layer; wherein an electrostatic force applied tothe second electrically conductive layer causes the free portion todisplace towards the first electrically conductive layer, therebyincreasing the capacitance of the capacitor; and an electronic deviceformed on the substrate.
 2. The circuit of claim 1, further comprising alayer of electrically insulating material disposed between the firstelectrically conductive layer and the anchor portion.
 3. The circuit ofclaim 1, further comprising a dielectric layer disposed on the firstelectrically conductive layer.
 4. The circuit of claim 1, furthercomprising a dielectric layer disposed on the second electricallyconductive layer.
 5. The circuit of claim 1, wherein the free portion isbiased away from the first electrically conductive layer along a radiusof curvature.
 6. The circuit of claim 5, wherein a tip of the firstportion of the first electrically conductive layer is tapered.
 7. Thecircuit structure of claim 1, wherein the electronic device comprises atleast one of a resistor, a capacitor and an inductor.
 8. The circuitstructure of claim 1, wherein the electronic device comprises at leastone of a transmission line and an antenna.
 9. The circuit structure ofclaim 1, wherein the electronic device comprises at least one of atransformer, a filter circuit, a phased-lock loop circuit, a heterodynecircuit, a mixer circuit, a delay line, a super heterodyne circuit, andan oscillator.
 10. The circuit structure of claim 1, wherein theelectronic device comprises at least one of a transistor and a diode.11. The circuit structure of claim 1, wherein the anchor portion isfixed to an insulating layer disposed on a portion of the firstelectrically conductive layer.
 12. A circuit structure, comprising: asubstrate; a capacitor comprising a first electrically conductive layerfixed to the substrate; a dielectric layer fixed to a portion of thefirst electrically conductive layer; and a second electricallyconductive layer comprising an anchor portion and a plurality of freeportions, the anchor portion being fixed to the dielectric layer,wherein a stress profile in the second electrically conductive layerbiases the free portions away from the dielectric layer; wherein anelectrostatic force applied to the second electrically conductive layercauses the free portions to displace towards the first electricallyconductive layer, thereby increasing the capacitance of the capacitor;and an electronic device formed on the substrate.
 13. The circuit ofclaim 12, wherein the free portions are disposed in rows adjacent to oneanother.
 14. The circuit of claim 12, wherein the free portions aredisposed in columns adjacent to one another.
 15. The circuit structureof claim 12, wherein the electronic device comprises at least one of aresistor, a capacitor and an inductor.
 16. The circuit structure ofclaim 12, wherein the electronic device comprises at least one of atransmission line and an antenna.
 17. The circuit structure of claim 12,wherein the electronic device comprises at least one of a transformer, afilter circuit, a phased-lock loop circuit, a heterodyne circuit, amixer circuit, a delay line, a super heterodyne circuit, and anoscillator.
 18. The circuit structure of claim 12, wherein theelectronic device comprises at least one of a transistor and a diode.19. A variable capacitor structure, comprising: a substrate; a firstelectrically conductive layer fixed to the substrate and a secondelectrically conductive layer comprising an anchor portion and a freeportion, the anchor portion being fixed to the substrate andelectrically insulated from the first electrically conductive layer,wherein a stress profile in the second electrically conductive layerbiases the free portion away from the first electrically conductivelayer; wherein an electrostatic force applied to the second electricallyconductive layer causes the free portion to displace towards the firstelectrically conductive layer, thereby increasing the capacitance of thecapacitor.
 20. The capacitor of claim 19, wherein the anchor portion isfixed to an insulating layer disposed on a portion of the firstelectrically conductive layer.
 21. The capacitor of claim 19, furthercomprising a dielectric layer disposed on the first electricallyconductive layer.
 22. The capacitor of claim 19, wherein the freeportion is biased away from the first electrically conductive layeralong a radius of curvature.
 23. The capacitor of claim 22, wherein atip of the first portion of the first electrically conductive layer istapered.
 24. The capacitor of claim 19, further comprising a layer ofelectrically insulating material disposed between the first electricallyconductive layer and the anchor portion.